Engine manufacturers incorporate valve train systems with variable valve control systems to improve operating and emissions performance of internal combustion engines. These variable valve control systems include systems to accomplish variable cam phasing, cylinder deactivation, and variable valve lift and duration. Distinct engine operating characteristics resulting from use of the variable valve system include improved combustion stability at idle, improved airflow through the engine over a range of engine operations corresponding to improvements in engine performance, and improved dilution tolerance in a combustion charge. Benefits of incorporating the variable valve system into an engine include improved fuel economy, improved torque at low engine speeds, lower engine cost and improved quality through elimination of external exhaust gas recirculation (‘EGR’) systems, and improved control of engine exhaust emissions.
A typical internal combustion engine is comprised of at least one cylinder containing a piston that is attached to a rotating crankshaft by a piston rod. The piston slides up and down the cylinder in response to combustion events that occur in a combustion chamber formed in the cylinder between the piston and a head. The head contains one or more intake valves to control the flow of air and fuel into the combustion chamber, and one or more exhaust valves that control the flow of exhaust gases out of the combustion chamber. A rotating camshaft opens and closes the intake and exhaust valves, and is synchronized with the position of each piston and the crankshaft. As an example of a variable valve system, a typical variable cam phasing system includes a variable cam phaser attached to an engine camshaft, and a cam position sensor that measures rotational position of the camshaft. The variable cam phasing system varies the opening and closing of each affected valve by varying angular position and rotation of the camshaft, relative to angular position and rotation of the crankshaft and each respective cylinder. An oil control valve diverts flow of pressurized engine oil to control the variable cam phaser, primarily based upon feedback from the cam position sensor. Typically an electronic engine controller controls this operation.
Timing, duration, and amplitude of valve opening affects mass of air that flows into an individual cylinder, thus affecting volumetric efficiency of the internal combustion engine. Fuel delivery to the internal combustion engine is typically determined by measuring or calculating mass air flow and determining an air/fuel ratio required to meet operator demand for performance and requirements for engine emissions. A quantity of fuel for delivery to each cylinder is determined based upon the combination of mass airflow and the required air/fuel ratio. A combustion charge is then created in each cylinder by delivering the quantity of fuel near the intake valve of the cylinder, or directly into the cylinder. This is known to one skilled in the art.
Performance of the variable valve control system, in terms of response time and ability to maintain the valve opening relative to piston position, may be affected by several system factors. These system factors include, for example, oil contamination, wear and viscosity, part-to-part variability caused by manufacturing tolerances, engine operating temperature, and component wear. These factors result in an inability of the controller to precisely control the variable valve control system, including a reduction in the range of motion of the valve. Any benefits derived from the variable valve control system can be compromised as a result.
By way of example, the engine controller uses the variable cam phasing system on air intake valves to open each valve early in the intake stroke to improve airflow into the cylinder and increase volumetric efficiency at low engine speeds. The result is improved engine torque at low speeds, allowing for improved vehicle acceleration. In typical current variable cam phasing systems, the system is calibrated based upon a known set of operating factors and a limited quantity of components. The controller is able to compensate for many of the effects caused by the system factors previously discussed (i.e. contamination, part-to-part variability, engine operating temperature, oil viscosity, and component wear) with feedback from the cam position sensor and exhaust gas sensors.
Pressurized oil required for operation of the variable valve control device is typically supplied from an engine oil system, using an oil control valve to divert oil flow. The engine oil system employs an oil pump powered by the engine. A typical system requires the engine oil system to provide a sufficient quantity of pressurized oil at 1.5 bar to effectively move the variable valve control device and achieve desired performance benefits. The oil pressure and flow to the variable valve control device is dependent upon variation in engine operating factors including speed and load, and the system factors mentioned previously. Response time and ability of the control valve to control the variable valve control system is dependent upon pressure and flow of oil through the oil control valve.
An engine designer specifies engine oil pump pumping capacity, in terms of flow and pressure, to ensure adequate pump performance to meet engine requirements, plus additional flow and pressure to operate the variable valve control device over the life of the engine. Operation of the variable valve control device includes an ability to move the device to a commanded position, and an ability to maintain the device at the commanded position. Moving the variable valve control device to the commanded position typically comprises a greater amount of flow than maintaining the variable valve control device at the commanded position. The controller uses the oil control valve to limit oil flow to the variable valve control device after it has been moved to the commanded position, and any remaining oil flow is diverted to other engine systems. Determination of the pumping capacity also includes compensation for effect of system factors, including oil contamination, wear and viscosity, part-to-part variability caused by manufacturing tolerances, engine operating temperature, and component wear. It is apparent that a portion of oil pumping capacity is unused over much of the life of the engine. This extra capacity adds unnecessary cost to the pump and consumes energy during operation.
Benefits of adding a variable valve control device must be balanced against increased system complexity and added cost to the base engine necessary to effectively operate the variable valve control device over the life of the engine. In cases wherein compromises are made in design of a system, benefits resulting from the system will not accrue, or will be offset by added cost to components of the system. Hence, there is a need for a method and system to effectively control a variable valve control system, while minimizing system complexity and added cost, and minimizing amount of energy consumed by the system.